This invention relates generally to electric storage batteries and more particularly to a battery construction, and method of manufacture thereof, suitable for use in implantable medical devices.
Rechargeable electric storage batteries are commercially available in a wide range of sizes for use in a variety of applications. As battery technology continues to improve, batteries find new applications which impose increasingly stringent specifications relating to physical size and performance. Thus, new technologies have yielded smaller and lighter weight batteries having longer storage lives and higher energy output capabilities enabling them to be used in an increasing range of applications, including medical applications, where, for example, the battery can be used in a medical device which is implanted in a patient""s body. Such medical devices can be used to monitor and/or treat various medical conditions.
Batteries for implantable medical devices are subject to very demanding requirements, including long useful life, high power output, low self-discharge rates, compact size, high reliability over a long time period, compatibility with the patient""s internal body chemistry, etc. Although various battery chemistries have been tried, lithium ion technology is generally accepted as the preferred chemistry for medical implant applications.
Such electric storage batteries are generally comprised of a tubular metal case enveloping an interior cavity which contains an electrode assembly surrounded by a suitable electrolyte. The electrode assembly generally comprises a plurality of positive electrode, negative electrode, and separator layers which are typically stacked and/or spirally wound to form a jellyroll. The positive electrode is generally formed of a metal substrate having positive active material coated on both faces of the substrate. Similarly, the negative electrode is formed of a metal substrate having negative active material coated on both faces of the substrate. In forming an electrode assembly, separator layers are interleaved between the positive and negative electrode layers to provide electrical isolation.
The present invention is directed to an electric storage battery incorporating one or more aspects described herein for enhancing battery reliability while minimizing battery size. In addition, the invention is directed to a method for efficiently manufacturing the battery at a relatively low cost.
In accordance with a first significant aspect of the invention, a feedthrough pin is provided which is directly physically and electrically connected to the inner end of an electrode substrate (e.g., positive), as by welding. The pin is used during the manufacturing process as an arbor to facilitate winding the layers to form an electrode assembly jellyroll. Additionally, in the fully manufactured battery, the pin extends through a battery case endcap and functions as one of the battery terminals. The battery case itself generally functions as the other battery terminal.
More particularly, in accordance with an exemplary preferred embodiment, the inner end of the positive electrode substrate is spot welded to the feedthrough pin to form an electrical connection. The substrate, e.g., aluminum, can be very thin, e.g., 0.02 mm, making it difficult to form a strong mechanical connection to the pin, which is preferably constructed of a low electrical resistance, highly corrosion resistant material, e.g., platinum iridium, and can have a diameter on the order of 0.40 mm. In order to mechanically reinforce the pin and secure the pin/substrate connection, a slotted C-shaped mandrel is provided. The mandrel is formed of electrically conductive material, e.g., titanium-6Al-4V, and is fitted around the pin, overlaying the pin/substrate connection. The mandrel is then preferably welded to both the pin and substrate. The mandrel slot defines a keyway for accommodating a drive key which can be driven to rotate the mandrel and pin to wind the electrode assembly layers to form the spiral jellyroll.
In accordance with a further significant aspect of the invention, the outer layer of the jellyroll is particularly configured to minimize the size, i.e., outer radius dimension, of the jellyroll. More particularly, in the exemplary preferred embodiment, the active material is removed from both faces of the negative electrode substrate adjacent its outer end. The thickness of each active material coat can be about 0.04 mm and the thickness of the negative substrate can be about 0.005 mm. By baring the outer end of the negative electrode substrate, it can be adhered directly, e.g., by an appropriate adhesive tape, to the next inner layer to close the jellyroll to while minimizing the roll outer radius dimension.
A battery case in accordance with the invention is comprised of a tubular case body having open first and second ends. The feedthrough pin preferably carries a first endcap physically secured to, but electrically insulated from, the pin. This first endcap is preferably secured to the case body, as by laser welding, to close the open first end and form a leak free seal. With the jellyroll mounted in the case and the first endcap sealed, the interior cavity can thereafter be filled with electrolyte from the open second end.
In accordance with a still further aspect of the invention, the jellyroll assembly is formed with a flexible electrically conductive tab extending from the negative electrode substrate for electrical connection to the battery case. In accordance with a preferred embodiment, the tab is welded to a second endcap which is in turn welded to the case. The tab is sufficiently flexible to enable the second endcap to close the case body second end after the interior cavity is filled with electrolyte via the open second end. In accordance with an exemplary preferred embodiment, the tab is welded to the inner face of the second endcap such that when the jellyroll is placed in the body, the tab locates the second endcap proximate to the body without obstructing the open second end. After electrolyte filling, the case body is sealed by bending the tab to position the second endcap across the body second end and then laser welding the endcap to the case body.